This invention relates to web cutting apparatus. It relates more particularly to cutting apparatus of the rotary type for cutting webs such as paper and the like transversely of the path of web travel.
Rotary knives have been used for many years to effect transverse cuts in moving webs including multi-ply and folded sheets. Such knives include an elongated rotary cylinder which supports an array of elongated radially outwardly projecting, longitudinally extending knife blades spaced apart around the periphery of the cylinder. The circumferential spacing of such blades determines the lengths of the sheets severed from the moving web.
Spaced parallel to the knife cylinder is an anvil cylinder. The rotary motion of the two cylinders brings the knife blades into the nip between the two cylinders, in turn, to effect cuts in the web moving through the nip. In many instances, provision is made in the apparatus for permitting the knife blades and anvils to be removed or adjustably positioned to cut different length sheets from the web and thereby accommodate the particular repeat pattern of printed material on the web. Rotary cutters such as this are disclosed, for example, in U.S. Pat. Nos. 3,555,948; 3,073,196; and 3,111,875.
In conventional rotary cutters, there is a tedious and time consuming "rolling in" process whereby the heights of the knife blades on the knife cylinder are adjusted to obtain the required amount of interference with the anvil cylinders to cut the web. Typically the knife blades are bolted along their lengths to fixtures mounted on the knife cylinder. The rolling in process involves an initial torquing of the blade-securing bolts with the blades set high and then advancing each knife blade into the nip so that it is displaced by the anvil to the point where its edge just kisses the corresponding surface of the anvil. The initial torque should provide sufficient resistance such that the blade is pushed away by the anvil only by the amount required to obtain the desired interference with the anvil. Then the bolts are tightened to maintain that blade setting. If the initial torquing operation is done just right, the desired amount of interference, typically only a few ten thousandths of an inch, is obtained. However, it takes a certain knack or skill to provide just the right amount of pretorque in a given case. More often than not the process has to be repeated more than once for each blade before the cutter is ready to operate.
If a particular knife blade has not been correctly positioned and is too close to its anvil during normal operation, the blade edge bears against the anvil with excessive force and becomes dull relatively quickly. As a result, the cutter has to be shut down relatively frequently while the blades are sharpened or replaced. On the other hand, if a blade is not close enough to its anvil in the nip between the two cylinders, the force developed at the instant of interference is less than that required to sever the web, e.g. 400-600 lbs. per lateral inch in the case of paper web. Resultantly, the apparatus does not cut the travelling web reliably, thereby causing downstream web-handling problems. Also the very fact that an interference between the knife and anvil is required to produce the force to cut the web produces problems in some applications. More particularly at the instant of interference, inherently the machine frame or the cylinders themselves are stressed and distorted. Therefore when the cutter is set up with its knife blades arranged close together, say, in closely spaced pairs to remove "bleed trims" between successive panels to be cut from the web, the parts distortions caused by the first blade interfering with its anvil persist long enough to upset the spatial relationship between the second blade and its anvil when that blade is rotated into the nip. As a result, the second cut is often defective, so that the bleed trim is not removed, or even worse, the partially severed trim strip, impaled on the usual strip removal pins, carries the leading edge of the web around the knife cylinder so that the apparatus has to be shut down.
Furthermore, even if all of the knife blades are properly rolled in, excessive wear and cutting problems can still arise because the machine parts have natural frequencies of vibration at certain machine speeds which often cause relative movement between the knife cylinder and the anvil cylinder that changes the interference between the knife edges and the anvils. At one instant, when a knife is opposite its anvil, the two may be moving toward one another so that the knife interferes too much with the anvil resulting in excessive blade wear. At another instant, the two cylinders may be moving away from one another so that a particular blade does not interfere enough with its anvil and the web is only partially cut. Thus as the cutter and other equipment in the line including the press are being brought up to speed, often one or another of these machine resonances is encountered which upsets the knife-anvil interference with the aforesaid consequences.
In addition, thermal effects sometimes upset the preset interference between blade and anvil. That is, as the apparatus starts and continues to operate, its various bearings and moving members heat up. This heat causes expansions of the machine frame elements and the cylinders which change the cylinder center-to-center distance. Resultantly, even if the blade is set correctly, in time the interference may increase to the point of becoming destructive to the blade edge or it may decrease so that the web is not severed correctly.
Further, in rotary cutters of this general type some provision must be made for removing trim strips or bleed trims that result from closely spaced transverse cuts in the web. Conventionally this has been done by providing a lengthwise series of ports in the knife cylinder between the blades. A vacuum is maintained at the ports as the web is cut to hold the strip. Then a positive pressure is applied to the ports to expell the strip at a location removed from the nip. This arrangement is not reliable enough especially with multiple ply webs. Resultantly some strips are not removed and are carried around the cylinder causing defective cuts. Also the valving and porting required are complex and therefore expensive to implement. Other prior machines use air loaded pins between the blades to eject the strips captured between the blades. Here again poor reliability is a problem.
Still another type apparatus has a perforating rule positioned between the knife blades whose teeth impale the strips and bear against a length of tape on the anvil cylinder at the nip. The strips are removed from the rule mechanically at a location away from the nip. With this arrangement, the type and thickness of the tape against which the rule teeth bear must be varied depending upon the type of web stock and the thickness and number of layers in the web. Therefore it requires a relatively long setup time. Still other cutters impale the trim strips on pins at the nip and then comb the strips from the pins with fixed fingers mounted at a location away from the nip. This arrangement is also complex and costly.